Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semi-conductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as P-type and N-type according to whether they comprise semi-conductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semi-conductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semi-conductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and inject holes or electrodes. For example, a P-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO level of the semi-conductive material can enhance hole injection and acceptance. In contrast, an N-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO level of the semi-conductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form thin film transistors. When an organic material is used as the semi-conductive material in such a device, it is known as an organic thin film transistor.
Various arrangements for organic thin film transistors are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semi-conductive material disposed therebetween in a channel region, a gate electrode disposed adjacent the semi-conductive material and a layer of insulting material disposed between the gate electrode and the semi-conductive material in the channel region.
An example of such an organic thin film transistor is shown in FIG. 1. The illustrated structure may be deposited on a substrate (not shown) and comprises source and drain electrodes 2, 4 which are spaced apart with a channel region 6 located therebetween. An organic semiconductor (OSC) 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4. An insulating layer 10 of dielectric material is deposited over the organic semi-conductor 8 and may extend over at least a portion of the source and drain electrodes 2, 4. Finally, a gate electrode 12 is deposited over the insulating layer 10. The gate electrode 12 is located over the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The structure described above is known as a top-gate organic thin film transistor as the gate is located on a top side of the device. Alternatively, it is also known to provide the gate on a bottom side of the device to form a so-called bottom-gate organic thin film transistor.
An example of such a bottom-gate organic thin film transistor is shown in FIG. 2. In order to show more clearly the relationship between the structures illustrated in FIGS. 1 and 2, like reference numerals have been used for corresponding parts. The bottom-gate structure illustrated in FIG. 2 comprises a gate electrode 12 deposited on a substrate 1 with an insulating layer 10 of dielectric material deposited thereover. Source and drain electrodes 2, 4 are deposited over the insulating layer 10 of dielectric material. The source and drain electrodes 2, 4 are spaced apart with a channel region 6 located therebetween over the gate electrode. An organic semiconductor (OSC) 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The conductivity of the channel can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage. The drain current that is achievable for a given voltage is dependent on the mobility of the charge carriers in the organic semiconductor in the active region of the device (channel between the source and drain electrodes). Thus, in order to achieve high drain currents with low operational voltages, organic thin film transistors must have an organic semiconductor which has highly mobile charge carriers in the channel region.
The application of organic thin film transistors is currently limited by the relatively low mobility of organic semiconductor materials. It has been found that one of the most effective means of improving mobility is to encourage the organic material to order and align. This minimizes intermolecular spacing and encourages inter-chain hopping which is the predominant conduction mechanism in organic semiconductors. The highest mobility organic semiconductor materials in thin film transistors show substantial ordering and crystallisation, which is evident from optical micrography and X-ray spectroscopy.
Techniques for enhancing crystallisation of the organic semiconductor in an organic thin film transistor include: (i) thermal annealing of the organic thin film transistor after deposition of the organic semiconductor; and (ii) designing the organic semiconductor molecules such that the organic semiconductor inherently has an increased ability to crystallize after deposition.
The present inventors have identified some problems with the aforementioned methods of enhancing crystallisation in organic thin film transistor devices. One problem with the thermal annealing technique is that the device must be heated. This can damage components of the device, increase energy costs for the manufacturer, and increase the processing time required to manufacture such devices. One problem with the molecular design route is that it is time consuming and expensive to design new molecules with increased ability to crystallize. Furthermore, modifying the molecular structure of the organic semiconductor can detrimentally affect the functional properties of the material in the resulting thin film transistor. Additionally, modifying the molecular structure of the organic semiconductor can detrimentally affect the processability of the material during manufacture of organic thin film transistors. For example, the solubility of the material can be affected such that the material becomes difficult to solution process using deposition techniques such as spin coating or ink jet printing.
The present inventors have identified yet further problems which are common to both the aforementioned techniques. One problem is that both techniques result in an increase in crystallisation throughout the organic semiconductor layer. The present inventors have realised that it may not be desirable to increase the crystallinity, and thus the conductivity, of the organic semiconductor in regions outside the active channel region as this may lead to current leakage at the sides of the device and shorting problems between underlying and overlying metallisation. As such, the present inventors have realised that it would be advantageous to provide a method of increasing crystallisation of the organic semiconductor only in the active channel region of an organic thin film transistor.
Furthermore, neither technique allows the orientation of the organic molecules to be readily controllable as the semiconductor crystallizes. As stated previously, inter-chain hopping is the predominant conduction mechanism in organic semiconductors. If the organic molecules crystallize in an orientation which is perpendicular to a direction from the source electrode to drain electrode, then the number of hops required for a charge carrier to move from the source electrode to the drain electrode may be increased thus reducing conductivity. The present inventors have thus realised that it would be advantageous to provide a method of increasing crytallisation which also encourages the organic molecules to align in a direction from the source electrode to the drain electrode so as to reduce the number of hops required for a charge carrier to move from the source to the drain.
It is one aim of embodiments of the present invention to provide a solution to one or more of the problem discussed above.